The following points highlight the top two barriers to animal distribution. The barriers are: 1. Extrinsic Barriers 2. Intrinsic Barriers.
Animal Distribution: Barrier # 1. Extrinsic Barriers:
Those barriers working from outside of an individual’s body of the population are extrinsic barriers.
Extrinsic barriers are generally of three types:
(a) Physiographic
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(b) Biotic and
(c) Climatic barriers.
(a) Physiographic Barriers:
Different’ physical and geographical obstacles, which act as barriers are included in this type.
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It can further be described under following heads:
(i) Topographical Barriers.
(ii) Large bodies of Water.
(iii) Large bodies of Land Masses.
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(iv) Impurity and Lack of Salinity of Sea Water.
(v) Vegetative Barriers.
(vi) Desert.
(vii) Pressure and
(viii) Ice mass.
(i) Topographical Barriers:
High and extensive mountain ranges limit the distribution of animals, e.g., Himalaya. This fange is situated more or less parallel to the equator. On the south of this range is hot and moist plains of India, with a very distinct tropical fauna, like elephant, tiger, etc.
This resembles the faunal type of Africa. Whereas on the northern part of the mountain the climate is colder and the fauna is more or less similar to that of Europe, like oxeny, gibbons, etc.
(ii) Large Bodies of Water:
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Giant river systems and oceans act as effective barriers for Tand animals, specially flightless animals- like amphibians, reptiles, etc. Fresh water fishes cannot survive in marine water and similarly marine animals cannot survive in fresh water.
However, during breeding period some fishes migrate from their normal habitat to other places. When fresh water fishes migrate to the saline water, it is called ‘catadromous migration’ (e.g. byne, eel) and when marine fishes migrate to the freshwater, it is called ‘anadromous migration’ (hilsa, sad, salmon, etc.).
In most of the cases a small amount of salt water often is an absolute barrier for amphibians. For this reason, the Pacific islands are completely uninhabited by amphibians, unless they are introduced by man, e.g., Hawaii.
In India, Brahmaputra and Ganges with their tributaries obstruct the distribution of some forest animals.
(iii) Large Land Masses:
Both freshwater and marine animals find large land masses as barrier to their distribution.
(iv) Impurity and Lack of Salinity in Sea Water:
Most of the corals are found in the clear sea basins, because they cannot survive in dirty or impure waters. For that reason extensive coral reefs or sponge-growing areas are never found near the mouth of large rivers, such as Ganges, Amazon, Orinoco or Mississippi, each of which bears a great load of seaward flowing sediments.
Similarly, there are some corals, sponges and brachiopods which cannot withstand slightest change in salinity of the sea water.
(v) Vegetative Barriers:
Large dense forest range acts as barriers, both for forest living animals and also for large terrestrial animals. The tree living arboreals require forest for their habit, habitat and food, e.g., monkeys and other primates. For them, plain land mass will act as barrier.
On the other hand larger terrestrial animals are incapable of penetrating the dense forest. During Pleistocene period, the elephants (Elephas and Mastodons) -could not travel to South America from North America due to the presence of dense forest in Mexico.
(vi) Desert:
The animals which require water to complete its life cycle cannot move through dry, hot desert, e.g., amphibia. Moreover, if other animals try’ to travel through the desert, they must possess special type of adaptive features.
The fauna of South and North Africa are totally distinct due to the presence of Sahara desert. Similarly, the Kalahari Desert is responsible for the different faunal composition of Central Africa and Cape of Good Hope.
(vii) Pressure:
The pressure gradient of sea water limits the distribution of sea animals at different depths. The pressure of water in the deep sea is maximum and it gradually decreases towards the surface. The deep sea adapted animals cannot come up due to low pressure, e.g., Heterosomata. On the other hand, surface dwellers and pelagic animals cannot go down due to heavy pressure.
(viii) Ice Mass:
During Pliestocene period, the Scandinavian ice mass and Alpine glacier came very nearer (about 480 km distance). This is of 480 kms become very cold, and acted as a barrier between Atlantic and Mediterranean areas.
As a result, European bears (Ursus arctors) were divided into two groups; one remained as U. arctors in the eastern part, while the other which went to the western part got transformed into a new species Ursus spelaeus.
(b) Biotic Barrier:
The biotic barriers include food, enemies and competition from other living organisms.
(i) Food:
Certain arthropods (Aphid) depend upon a particular species of plant for their food, shelter and breeding place. Hence, absence of that plant in a particular area acts as a barrier for the distribution of such arthropods. The fact is same for arboreal frugivorous animals and also for carnivores, e.g., red panda, giant panda, etc.
(ii) Enemies:
Predators and parasites are completely dependent on specific prey and hosts, respectively, for their food and habitat. Therefore, for prey and host respective enemies will act as barriers. On the other hand, both predators and obligatory parasites could not survive in the absence of prey and host.
But prey and host try to avoid their predators, because of their harmful nature. The presence of an external parasite tse tse fly in certain part of Africa prevents the distribution of ungulates. Tsetse fly is the carrier of a parasite, Trypanosoma, which causes fatal disease sleeping sickness in ungulates.
(iii) Competition:
Inter-specific competition for food and space forces weaker species to move from one place to and other. But due to appearance of new competitor in the new place, this weaker species may not survive. In some places of Australia, appearance of rabbits diminishes the mursupials in their original home land.
(c) Climatic Barriers:
The abiotic factors, which can act as barrier to animal distribution are considered under this group.
These are:
(i) Temperature,
(ii) Rainfall,
(iii) Solar radiation and
(iv) Photoperiodism.
(i) Temperature:
Each species has a limited range of temperature tolerance, e.g., Rotifers appear in the ponds when temperature is below 24°C and disappears when it rises above 24°C. Coral reefs require minimum 21 °C temperature for their existence.
They are completely absent from cold water regions of the world. On the earth, temperature varies greatly from region to region. Therefore, an animal of a specific’ temperature tolerance cannot move uniformly throughout the earth.
(ii) Rainfall:
Excessive rainfall results in the formation of water bodies and moisture, which prevent the distribution of land animals. Again in low rainfall, trees will not grow. So tree living (arboreal) animals cannot survive.
Plants and animals are classified on an ecological basis into hydrocoles’ or water loving, ‘merocoles’ or those living in places with neither excess nor deficiency of water, and ‘xerocoles’ or those which live in dry places.
(iii) Solar Radiation:
Excessive solar radiation enhances evaporation. Some animals cannot survive in high evaporating area. They become confined to the moist shady areas under rock or in the forest or cave.
(iv) Photoperiodism:
Photoperiod is remarkably different in certain places in different seasons. This photoperiodic change forces some birds to migrate. Those animals which are flightless cannot inhabit the areas of fluctuating photoperiod.
Animal Distribution: Barrier # 2. Intrinsic Barriers:
The barriers which result from the nature and behaviour of an animal is called intrinsic barrier. Some of them are described below.
(a) Philopatry:
Some animals are by nature lazy and do not like to move from one place to another. Most higher animals possess this nature. Rovanda (Zosterpos rendovae), a bird, never moves from its native area and try to withstand all calamities.
(b) Homing Instinct:
“Most of the animals, though are capable of going distant places, never do so. They always return back to their native area.
(c) Parental Care:
Those animals having a long period of parental care, cannot move a long distance, because when the young’s become free, they are habituated to their mother’s land.
(d) Habitat Selection:
Each migrating animal, always tries to select an area similar to their homeland. This causes elimination from the new area. Thus, the presence or absence of barriers causes an animal to get isolated geographically. These geographically isolated animals gradually transform into new species in their new area. Therefore, barrier is considered as a reason for speciation.